1. Field of the Invention
The present invention relates to a method of manufacturing a carcass structure for vehicle wheel tyres, comprising the steps of: making at least one carcass ply by deposition of elongated sections circumferentially distributed on a toroidal support, each of said elongated sections extending in a U-shaped configuration around the cross-section outline of the toroidal support, to define two side portions mutually spaced apart in an axial direction, and one crown portion extending at a radially outer position between the side portions; applying annular reinforcing structures to a region close to the inner circumferential edges of said at least one carcass ply.
The present invention also relates to a carcass structure for vehicle wheel tyres to be obtained by the above method, said carcass structure comprising: at least one carcass ply comprising elongated sections circumferentially distributed around a geometric rotation axis, each of said elongated sections extending in a U-shaped configuration around the cross-section outline of the carcass structure, to define two side portions mutually spaced apart in an axial direction, and one crown portion extending at a radially outer position between the side portions; at least two annular reinforcing structures located close to respective inner circumferential edges of said at least one carcass ply.
2. Description of the Related Art
Manufacture of tyres for vehicle wheels involves formation of a carcass structure essentially consisting of one or more carcass plies substantially shaped in a toroidal configuration and having their axially-opposite side edges in engagement with respective annular circumferentially-inextensible reinforcing elements, usually referred to as “bead cores”.
Applied to the carcass structure, at a circumferentially outer position, is a belt structure comprising one or more belt strips in the form of a closed ring, essentially made up of textile or metallic cords suitably oriented with respect to each other and the cords belonging to the adjacent carcass plies.
Then a tread band usually consisting of a strip of elastomer material of appropriate thickness is applied to the belt structure at a circumferentially external position thereof.
It is to point out that, to the aims of the present invention, by the term “elastomer material” it is intended a rubber blend in its entirety, that is the assembly made up of at least one base polymer suitably amalgamated with reinforcing fillers and/or process additives of various types.
Finally, to the opposite sides of the tyre being manufactured a pair of sidewalls are applied, each of them covering a side portion of the tyre included between a so-called shoulder region, located close to the corresponding side edge, of the tread band, and a so-called bead located at the corresponding bead core.
In accordance with traditional production methods, essentially the above listed tyre components are first made separately from each other, to be then assembled during a tyre-manufacturing step.
For instance, for making the carcass ply or plies to be associated with the bead cores to form the carcass structure, production of a rubberized fabric comprising longitudinally-disposed continuous textile or metallic cords, through an extrusion and/or calendering process, is first required. This rubberized fabric is submitted to a transverse-cutting operation to produce lengths of predetermined sizes that are subsequently joined together so as to give rise to a continuous ribbon-like semifinished product having transversely-disposed parallel cords.
Then this article of manufacture is to be cut into sections the length of which is correlated with the circumferential extension of the carcass to be made.
Production methods have been also proposed which, instead of resorting to the production of semifinished products, make the carcass structure directly during the tyre-manufacturing step.
For instance, U.S. Pat. No. 5,453,140 herein quoted as an example of the most pertinent state of the art, discloses a method and an apparatus forming a carcass ply starting from a single cord previously wound around a reel.
According to the method and apparatus described in the above patent, at each operating cycle of the apparatus the cord drawn from the reel by power-driven idler rollers and maintained taut by a pneumatic tensioning system is cut to size to obtain a section of preestablished length.
The cord section is picked up by a pick-up element mounted on a belt looped around power-driven pulleys, to be then laid down transversely on the outer surface of a toroidal support.
The section ends are then engaged by bending members of the belt type operating on opposite sides of the toroidal support for radially applying the cord section to the toroidal support by means of slide elements acting like fingers along the side portions of the section.
Repetition of the above described operating cycle leads to deposition of a number of cord sections circumferentially disposed in side by side relationship until the whole circumferential extension of the toroidal support has been covered.
Necessarily, the toroidal support is previously coated with a layer of raw rubber having a dual function, i.e. that of adhering to the cords deposited thereon so as to conveniently hold them in a fixed positioning, and that of constituting an air-proof inner liner in the finished tyre.
Tyres obtained by this production method have a carcass structure in which the carcass ply or plies consist of individual cords each having two side portions axially spaced apart from each other and oriented radially of the rotation axis of the tyre, and one crown portion extending at a radially outer position between the side portions.
Within the scope of the carcass structure manufacture, it is also known that close to each of the tyre beads, the opposite ends of the individual cords forming a carcass ply are placed, in an alternating sequence, at axially opposite positions relative to an annular anchoring element constituting said bead core, made in the form of a crown consisting of thread coils radially superposed upon each other, as disclosed in Patent EP 0 664 231 and in U.S. Pat. No. 5,702,548.
In the above quoted art however, all cords forming the carcass ply or plies are substantially disposed along the neutral axis of resistance to bending of the respective bead. Under this circumstance, the structural strength of the beads must necessarily rely on the stiffness of the filling inserts of very hard elastomer material incorporated into the bead structure, the behaviour of which is affected by temperature changes due both to environmental factors and stresses produced during usual operation.
Proposed in Patent FR 384 231 is accomplishment of a carcass structure by deposition on a toroidal support of a series of rectangular bands of rubberized fabric circumferentially arranged close to each other and disposed in radial planes relative to the geometric axis of the support drum itself. Deposition of the bands is carried out in such a manner that the end flaps of two non-consecutive bands are partly covered with the end flaps of the band interposed between them. Spaces existing between the end flaps covered with the bands are occupied by trapezoidal inserts applied to the end flaps of the band overlapping the end flaps themselves. Deposition of the bands is carried out in different superposed layers, the number of which is correlated with the thickness to be given to the carcass structure. The presence of said trapezoidal inserts causes thickening of the carcass structure at the bead regions, so that a thickness which is twice that detectable at the crown is achieved.
In U.S. Pat. No. 4,248,287 a method is disclosed according to which for formation of the carcass structure it is provided that a plurality of layers each formed of radial strips made up of rubberized threads and circumferentially disposed in side by side relationship should be laid down on a toroidal drum. When deposition is completed, two bead cores are applied to the bead region and the end flaps of the carcass layers formed by the radial strips are turned up around them.
The Applicant has found that important advantages can be achieved both in terms of simplification of the production processes and in terms of improvement of the behavioural features of the tyre, if the carcass ply or plies are made by conveniently laying down strip-like sections on a toroidal support, each of said sections comprising a plurality of cords parallel to each other which are incorporated into an elastomer layer.
In this connection the Applicant has already developed different manufacturing methods being the object of respective European Patent Applications.
For instance, in European Patent Applications No. 97830731.2 and No. 97830733.8 a manufacturing method and a tyre are respectively described in which the carcass structure is obtained by making a first and a second carcass plies each obtained by strip-like sections laid down sequentially in mutual circumferential side by side relationship.
Tyres obtained in accordance with the description of these patent applications have the end portions of the strip-like sections belonging to the first and second carcass plies disposed on opposite sides respectively relative to the annular reinforcing structures of the bead.
This expedient, in combination with the respectively crossed orientation of the strip-like sections belonging to each of the plies offers important advantages in terms of structural strength of the tyre close to the beads and sidewalls.
In the European Patent Application No. 98830472.1 in the name of the same Applicant as well, accomplishment of a carcass ply is proposed which is made by laying down a first and a second series of strip-like sections in an alternating sequence, in which sections belonging to the first and second series terminate on respectively opposite sides relative to the reinforcing structures at the beads.
Thus, advantages can be achieved in terms of structural strength at the tyre beads and sidewalls even in the presence of a single carcass ply.
In the European Patent Application No. 98830662.7, still in the name of the same Applicant, the presence of two carcass plies each formed of two series of strip-like sections laid down in an alternating sequence is provided. The sections of each ply have a crossed orientation relative to those of the other ply and terminate at respectively opposite sides of a respective portion of the reinforcing structure at the bead, to give the tyre exceptional qualities in terms of mechanical resistance to stresses produced during the acceleration and deceleration steps, as well as during a slip running.